(AP) - A Republican congressman from western Michigan has decided not to run for the U.S. Senate seat opening with the retirement of Democrat Carl Levin.

Justin Amash of Kent County's Cascade Township confirmed on Twitter Tuesday night that he won't run. The libertarian conservative has gained a higher profile in the House with a challenge to the National Security Agency's collection of Americans' phone records.

Amash's announcement further clears the way for former Secretary of State Terri Lynn Land to proceed with her campaign.

Land is the only declared Republican candidate. Democrats have coalesced behind U.S. Rep. Gary Peters of Bloomfield Township. The other known Republican still interested in running is Ypsilanti cardiologist Rob Steele, who's had backing from the tea party.